What is Body Mass Index (BMI)?

Body Mass Index (or BMI) is a number that is calculated based on your weight and height. You can use BMI as a simple check to see if you are classed as underweight, normal weight, overweight or obese. A normal BMI range is between 18.5 and 24.9, which is thought to reflect a healthy weight for the typical adult.

BMI is used by healthcare professionals as a quick and non-invasive method to assess a person’s body weight. It is a useful starting point for conversations about weight management and health risks associated with being underweight or overweight. These conversations can guide individuals towards a healthier diet, more physical activity and a healthier lifestyle.

The BMI formula for adults is simple. You simply divide your weight in kilograms by the square of your height. This calculation provides your BMI in units of kg/m². If measurements are taken in pounds and inches, you will first need to convert them to the metric units.

What is a healthy BMI?

A healthy Body Mass Index (BMI) ranges between 18.5 and 24.9. People falling within this range are typically considered to be a healthy weight for their height.

The World Health Organization (WHO) classifies Body Mass Index into several categories to help individuals understand their body weight status. These categories are:

1. Underweight: BMI less than 18.5

2. Normal weight: BMI between 18.5 and 24.9

3. Overweight: BMI between 25 and 29.9

4. Obesity class I: BMI between 30 and 34.9

5. Obesity class II: BMI between 35 and 39.9

6. Obesity class III: BMI of 40 or higher

It’s important to remember that BMI is a screening tool and not a diagnostic test. For an accurate health assessment, it’s best to consult with a healthcare provider.

Risk factors of a high BMI

According to the World Health Organization, being overweight or obese increases your risk for a number of health issues. These include cardiovascular diseases, such as heart disease and stroke, diabetes, musculoskeletal disorders like osteoarthritis, and some cancers including breast, ovarian, prostate, liver, gallbladder, kidney and colon cancer.

Maintaining a healthy weight can lower the risk of these conditions and enhance your overall quality of life. A balanced diet, paired with regular exercise, are key to achieving and maintaining a healthy weight.

Is BMI accurate?

BMI is a useful tool for assessing a person’s weight status. It’s important to say, though, that BMI should be just one aspect of health assessment, and it’s important to consider other risk factors like your muscle-to-fat ratio, waist circumference and lifestyle.

Despite its usefulness for most people, a limitation of BMI is that it does not distinguish between fat and muscle. Muscle tissue is denser than fat, so it weighs more than fat for the same amount of ‘volume’. Athletes and muscular individuals may have a high BMI even though they are not considered overweight or obese. This is why it is essential to consider other measures and factors in assessing your weight.

Body composition tests can accurately measure your lean mass and fat mass, providing more detailed and personalised insight into your health. You should always consult a doctor or suitably-qualified medical professional if you are concerned about your weight.

BMI calculator

A BMI calculator is a tool that outputs your BMI score based on your height and weight. It will also provide your weight category, based on the standard reference ranges defined by the WHO, so you can easily gauge if you might be underweight, healthy weight or overweight.

For adults, BMI calculators use the standard formula (weight divided by the square of your height).

For children and teenagers

For children and teenagers, the BMI calculation uses a different method. This is because children and teenagers grow at different rates based on their age and gender.

Rather than using a formula, BMI is instead calculated relative to population averages for each age group and gender. The output is given as a centile – a number between 0 and 100 – that compares a child to others in the same group. A score between 2 and 91 is considered a healthy weight.

To calculate BMI for a child aged between 2 and 17, use the NHS’s children and teens BMI calculator.